Have it evaluated. Callusing is usually secondary to mechanical pressure(something compressing the skin against bone). A foreign body is also a possibility. See your podiatrist for evaluation and treatment options.

Painful calluses. There will often be a focal spot of increased pressure in a callus, usually over a bony prominence. This will be seen as a "deep spot" within the body of the callus and gets to be quite painful. Conservative care consists of reducing the callus; add an orthotic device with an accommodation to further reduce the pressure on the spot. Surgery sometimes to correct the underlying bone deformity.